Electrical motors are well known. Although there is a host of electrical motor designs, the standard 3-phase AC induction motor is the established workhorse of industry. In its essential aspects this electrical motor is a direct electromechanical analogue of the 3-phase generators utilized in power plants. In this regard, the 3-phase induction motor is a natural fit for the type of AC power available for its operation. When running directly from 3-phase AC grid power, no conceivable motor scheme is more adaptable or efficient than the 3-phase AC induction motor.
A “drive” (sometimes called a “controller”) provides the excitation that causes an electrical motor to operate. Conventional variable frequency AC drives attempt to duplicate the voltage/current characteristics of 3-phase power to obtain the most efficient performance from an AC induction motor. In so doing the drive must create three sinusoidal waveforms, each mutually displaced from the other two by 120 degrees, from a DC power source located either external to the drive or internally as the “DC link”. Viewed overall, an AC motor is made to run from a DC source where the drive mediates the interface between two different formats of electrical energy.
The so-called “brushless DC motor” is basically the same machine as the AC induction motor except that the squirrel-cage rotor of the latter is replaced with a permanent magnet rotor. Each type uses an identical 3-phase stator and each requires the same 3-phase AC drive power to the stator. In some respects the brushless DC motor is more suitable to the electronic drive because large reactive currents inherent in the AC induction motor are largely non-existent in the brushless DC version, which simplifies drive design and reduces some losses. However, because of the cost and difficulty involved in managing large quantities of permanent magnet material, the brushless DC motor has not proven commercially viable in higher horsepower ranges.
A present problem with electrical motor systems is the difficulty of merging motor and drive in a simple, easily assembled, efficient, and economical combination. Typically, a drive, in synthesizing an AC waveform to excite an electrical motor from a DC power source employs expensive, high capacity switching devices such as IGBTs (Insulated Gate Bipolar Transistors) to generate a high frequency PWM (Pulse Width Modulation) waveform. Such drives are characterized by complexity in a customized design that generates and accommodates a PWM waveform, cost in circuit implementation, and inefficiency resulting from switching losses associated with PWM. Current drive design and construction result in a sizable piece of equipment that consumes resources for storage, shipment, and installation.
In contrast, in this invention, electrical motor and drive are merged into a single, inexpensive, highly effective, integrated design. Rather than accommodate the standard AC induction motor to a large, expensive DC power source, the motor itself is modified to make it compatible to a DC power input, whether directly from a battery pack or fuel cell, or from rectified AC power. In this regard, the motor is provided with a sizable air gap between the stator and the rotor in order to impart a pronounced magnetic inductance to the motor itself. This inductance is placed in series with capacitance to constitute a resonant circuit which is caused to oscillate when DC power is switched to it. The waveform or waveforms produced by the oscillation of the resonant circuit excite the motor.